Most current vaccines target diseases that cause acute infections and are then cleared by the immune system. In vaccine development, a significant inclusion criterion has been the extent to which a given immunogen is able to generate a strong T-cell response. However, this strategy has not been optimal for persistent, or latent infections, such as those caused by human cytomegalovirus (HCMV). In fact, some immunogens generate very strong T-cell responses, but are ineffective as vaccines against viruses that cause persistent or latent infections.
Vaccines are often targeted against immunodominant proteins of the virus. That is, vaccines often use the same antigens that are recognized and targeted by the immune system of a healthy host. HCMV candidate vaccines in preclinical and clinical testing are focused on eliciting CD8+ T cell or neutralizing antibody responses toward HCMV proteins that are known to be targeted during HCMV infection of the immunocompetent, healthy host. The HCMV targets for CD8+ T cell-mediated immunity being tested are most commonly the immunodominant UL83-pp 65 and IE1-pp72 proteins. While CD8+ T cells specific for these viral proteins are believed to be protective, recent results in an animal model suggest that CD8+ T cells against immunodominant CMV antigens may not provide any protection despite their high numbers in the infected host.
In the United States, approximately 40,000 newborns are congenitally infected with human cytomegalovirus (HCMV) annually. For many years, it has been recognized that human cytomegalovirus (HCMV) is efficiently transmitted to the fetus during pregnancy, with 0.5 to 2.5% of all newborns showing evidence of congenital infection. Unfortunately, the in utero infection is not benign, and 5 to 10% of the congenitally infected infants will be symptomatic at birth, with serious neurological defects. (for review, see (67)). Of the 5% to 10% that are symptomatic at birth, most develop sequelae such as microcephaly, sensorineural hearing loss, optic atrophy and chorioretinitis, and motor disabilities. Even the infected children who appear asymptomatic at birth are at high risk, as 10 to 15% of these children will show varying degrees of neurological damage later in life. The problem is intensified by the large increase in the number of young children in day care centers as the transmission rate of HCMV among children in these centers is high, and these children will frequently transmit the virus to their seronegative mothers or day care providers. The annual seroconversion rate for women with infected children is 30% as compared to a 3% rate for women with uninfected children. Moreover, immunization with the Towne strain of HCMV, which has been tested as a potential vaccine, did not significantly decrease the transmission rate. Since it is this group of women who most commonly will be pregnant, the risk to the newborn is significant. While the serological status of the mother positively correlates with protection of the newborn from disease, recent evidence strongly suggests that prior maternal immunity is not completely protective against neonatal disease from recurrent infection or infection with a different HCMV strain (4, 21). The devastating consequences of in utero infection make it imperative to develop an effective and safe vaccine that will prevent both acute infection and the establishment of latency. In addition to the effect in newborns, HCMV disease in other populations, such as, for example, transplant recipients, including both solid tissue and bone marrow transplants, can be quite severe, and has gained greater attention as the number of transplants has significantly increased.
Recovery from HCMV disease correlates with a cellular immune response rather than the presence of CMV-neutralizing antibodies (59, 70). Initially, it appeared that most of the HCMV-specific CTLs were directed against the HCMV UL83 (pp 65) matrix phosphoprotein with relatively low levels of CTLs specific for the HCMV IE1 72-kDa protein (a functional homolog of the MCMV gB) and the structural glycoprotein B (1, 5, 23, 28, 55, 78). Subsequently, it was found that the frequency of CD8+ CTLs directed against IE1 is similar to that against pp 65 (30, 43), and that multiple HCMV proteins are potential targets for CD8 T cells (14, 50).
An effective vaccine against HCMV disease has been an elusive goal for many years, even though many of the antigenic targets of the neutralizing antibody and CD8+ T cell responses have been identified (for reviews, see (26, 67)). Clinical trials using the tissue culture-passaged Towne strain was found to induce both neutralizing antibodies and CTLs and provided limited protection against severe disease in transplant recipients and in volunteers given a low dose HCMV challenge, but failed to prevent infection in women exposed to young children shedding HCMV. The envelope glycoprotein B (gB) has been the basis for virus neutralizing antibody inducing vaccines, both as a subunit vaccine (adjuvanted with MF59) as well as a recombinant replication deficient canarypox vector ALVAC-CMV (gB). Both vaccines were found in clinical trials to be well tolerated, and although the subunit gB vaccine was found to elicit high levels of HCMV neutralizing antibodies in seronegative volunteers, ALVAC-CMV (gB) was only able to elicit neutralizing antibodies after subsequent boosting with Towne. Preliminary results have been obtained following vaccination of seronegative subjects with the pp 65 expressing ALVAC-CMV (pp 65) vector, as strong pp 65-specific CTL levels were elicited as well as CTL precursor frequencies similar to those found in HCMV seropositive subjects. Other vaccination approaches that have undergone preclinical testing in mice include plasmid DNA (pDNA) encoding gB or pp 65, a peptide of the conserved CD8+ T cell epitope of pp 65, dense bodies, and more recently a recombinant vaccinia virus Ankara that expresses gB (2, 16, 17, 46, 69, 91). The key question; however, is whether they will protect against infection in seronegative individuals.
There is a need for an HCMV vaccine that can prevent HCMV infection, and that could limit HCMV replication, and possibly vertical transmission from mother to fetus or viral dissemination and disease in the transplant recipient.
Herpes simplex virus type 2 (HSV-2) is a medically important pathogen worldwide, with a seroprevalence rate that has been increasing in the US over the last two decades. HSV-2 infects between 10 and 50% of the population worldwide, and in the US, it is estimated that 20% of the population is infected (for review, see (92)). A unique property of the herpesviruses is that they can enter latency, a state characterized by the absence of infectious virus and limited viral gene expression. In response to various stimuli, the virus reactivates, replicates, and produces infectious virions. HSV-2 infection is usually initiated following sexual contact of a seronegative individual with someone who is shedding infectious virus. The primary infection of genital, perigenital, or anal mucosal skin sites is followed by transmission most commonly to the sacral ganglia where the virus establishes latency. Reactivation from the ganglia then leads to infection and viral shedding in the vagina or skin of the penis. The frequent reactivation of genital herpes not only is a source of physical discomfort and psychological stress, but also can cause serious disease in the newborn, which often leads to death. A major problem in controlling sexual transmission is that shedding of virus may be asymptomatic, and the incidence of HSV-2 infections continues to increase. Although antivirals are available, the lifelong persistence of this virus provides a strong impetus for the development of a vaccine that will prevent infection and the establishment of latency.
There is a need for the development of an effective vaccine that prevents infection and the establishment of latency, thus eliminating the possibility of recurrence. While most vaccine strategies to date have failed in human clinical trials, the immunological data gained over the years have shown that HSV-2 infection is controlled by both innate and adaptive immune responses, with CD8 and CD4 T cells playing a major role in viral clearance from the lesion. To provide the basis for a protective T cell based vaccine, recent work has identified many of the HSV-2 proteins that are primed by infection. However, the cellular and antibody responses to HSV-2 infection are not sufficient to provide sterilizing immunity and protection against recurrent infection and viral shedding. Thus, immune responses generated by a successful vaccine must be more effective than natural immunity.